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THE SATIN MOTH 

AN INTRODUCED ENEMY OF 
POPLARS AND WILLOWS 

A. F. BURGESS 

Entomologist in Charge of Gipsy Molh and 
Brown-tail Moth Investigations 




Rows of 25- Year-Old Carolina Poplars. A favorite food of the Satin Moth 

UNITED STATES DEPARTMENT OF AGRICULTURE 
DEPARTMENT CIRCULAR 167 



Contribution from the Bureau of Entomology 
L. 0. HOWARD, Chief 



Washington, D. C. 



Issued May. 1921 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1921 



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( p.uARY OF CONGR686 | 

OCT 3 1933 



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THE SATIN MOTH: AN INTRODUCED 

ENEMY OF POPLARS AND WILLOWS. 



DISCOVERY OF AN UNWELCOME IMPORTATION. 

The satin moth (Stilpnotia salicis L.), a well-known insect in 
Europe, was first reported in the United States about July 1, 1920. A 
crew of men employed by the Metropolitan Park Department found 
a number of Carolina poplar trees located on the Fellsway at the 
Maiden and Medford city line just north of Boston that were being- 
defoliated. The gipsy moth was first suspected as the probable cause 
of the trouble, but specimens were later collected by the foreman and 
turned over to Mr. A. N. Habberley, the park superintendent. As he 
did not recognize the caterpillars, he brought them to the Gipsy 
Moth Laboratory at Melrose Highlands. They did not seem to be a 
native species, and accordingly Mr. S. S. Crossman, one of the assist- 
ants at the laboratory, visited the insect collections in Boston and 
Cambridge and was able to identify the insect as the satin moth by 
comparison with larva? in the collection of the Museum of Compara- 
tive Zoology at Cambridge, Mass. Confirmation of the determination 
was later secured from specialists at the United States National 
Museum. As this insect is closely related to the gipsy moth and the 
brown-tail moth, having habits which are similar in some respects to 
those of each of these species, considerable anxiety was felt as to the 
amount of damage it might cause. 

The caterpillars were nearly full-grown at the time they were 
found and some of the trees were badly defoliated. A hasty examina- 
tion showed that the most severe defoliation was in the region where 
the caterpillars were first reported, and several trees in a small group 
of Lombardy poplars were completely stripped. (Fig. 1.) A row of 
large California poplars directly in the rear of a number of houses 
were being heavily eaten and the owners complained bitterly about 
the superabundance of caterpillars which invaded the houses and 
made life extremely disagreeable, as they frequently crawled upon 
the tables and even on the beds, and in spite of all the care that 
could be taken it seemed impossible to keep them out. The sides of 
these houses nearest these trees were heavily stocked with larvae, par- 
ticularly under the eaves and clapboards, and in a few days many of 
them began to pupate. 

CONTROL WORK UNDERTAKEN. 

As soon as the identity of the species was determined, arrange- 
ments were made with the superintendent of parks and with the 
State forester's office, which in Massachusetts has charge of the gipsy- 
moth control work, to cooperate with the Bureau of Entomology. 
No special funds were available for the purpose, but work was under- 
taken to reduce the damage and danger of spread as much as possible 

3 



4 Department Circular 167, U. S. Dept. of Agriculture. 



until more complete information as to the nature and possibilities 
for destructiveness of the insect could be determined. 

Arrangements were made that the Park Department should de- 
stroy as many of the caterpillars and pupse as possible on the trees 
along the Parkway which were under its control ; that the State 



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Fig. 1. — Row of Lombardy poplar trees, the three on the left completely defoliated 
by the satin moth (Stilpnotia mlicis) ; the others partially defoliated. July, 1920. 

forester's office, should examine the residential, property in and 
around the center of infestation and destroy the insects thereon, and 
that the Bureau of Entomology of the United States Department of 
Agriculture should scout as much as possible of the outlying terri- 
tory to determine the extent to which the insect had spread, secure 



The Satin Moth. 5 

field and laboratory data on the habits and life history of the insect, 
and obtain all possible information on the best methods of control. 

Accordingly, the Park Department immediately sprayed the trees 
along the Parkway in and about the worst infested part of the terri- 
tory ; but, owing to the fact that most of the larvae were nearly full- 
grown, the treatment was not very effective. It also employed a crew 
of trained gipsy-moth men to patrol the trees and crush all the 
larvae within reach. Later these men were kept busy crushing pupa? 
and treating the egg masses with creosote. 

At the same time assistants from the State forester's moth force 
carried on similar work on the privately-owned grounds adjoining 
the Parkway and large numbers of the insects were destroyed. 

The scouts supplied by the Bureau of Entomology commenced an 
examination of the surrounding territory, particularly where poplar 
or willow trees were present ; but as scattered, though small, infesta- 
tions were found each day, they began working in towns more remote 
from the center. 

Investigations were also begun by experts from the Gipsy Moth 
Laboratory, and such time as could be spared during the busiest sea- 
son of the year was devoted to a study of the behavior of the insect 
in the field, its food plants and enemies. 

As soon as the moths began to emerge as many as possible were 
crushed, but as quantities of the caterpillars pupated within curled- 
up leaves on the trees, the method was only partially effective. Both 
sexes of the moth fly readily and are attracted to brilliant lights. 
Observations at night materially assisted in securing records of the 
spread of the insect. 

THE INSECT, ITS DEVELOPMENT AND HABITS. 

Daring the summer of 1920 the following facts were learned con- 
cerning the insect : 

THE MOTHS. 

The insect derives its name from the white, satinlike appearance 
of the moths (fig. 2), which are larger than most of the white moths 
native to New England and have no colored markings on the wings. 

The males are 15 to 20 mm. in length and the females 20 to 25. 
The head, thorax, and abdomen are black but densely covered with 
long white hairs, so that they resemble the color of the wings, which, 
as stated, have a satiny luster. The eyes are black. The legs are 
black and the femora are densely covered with long white hairs. The 
tibiae and tarsi are sparsely covered with white hairs and ringed with 
white overlapping scales which give the appearance of alternate 
bands of black and white. The dorsal side of the antennal shaft is 
white, the pectinations brown to black. The expanse of the fore- 
wings is from 35 to 55 mm. The females usually are slightly larger 
than the males. 

THE EGGS. 

The eggs are laid in somewhat elongated patches 13 to 22 mm. in 
length and covered with a white secretion which holds them together 



6 Department Circular 167, U. S. Dept. of Agriculture. 

in the cluster. (Fig. 3, Z?, C.) They are very conspicuous and are 
easily distinguished from the egg masses of other moths. Dissections 
show that a female usually produces about 550 eggs, although some 




Fig. 2. — The satin moth ; Male above, female below. Enlarged. 

individuals lay a larger number. If the female is disturbed she fre- 
quently deposits more than one cluster. She may fly from the place 
where egg laying was begun and continue the operation in another 
locality. Although the eggs are usually laid on the underside of the 



The Satin Moth. 




Fig. 3. — Tbe satin moth : a, Poplar leaf partially skeletonized by small larvae tsmall 
larva shown below leaf) ; b, egg cluster on leaf; c, egg cluster on bark. 



8 Department Circular 167, U. S. Dept. of Agriculture. 

leaves and on the branches and trunks of the trees, they are sometimes 
deposited on grass, weeds, or stones, and even on the ground if no 
more convenient place is available. 

In 1920 the first eggs were observed in the field July 6, and an 
unhatched cluster was found as late as August 10. This indicates 
that e^cr laying sometimes continues for about a month. Most of 
the eggs were deposited about the middle of July. 

Hatching takes place in about 15 days after the eggs are laid, but 
this period is frequently shorter if the weather is hot. 

THE LARWE. 

First-stage larvae. — Newly hatched larva? are 2 mm. long. The 
head is brownish black, provided on front with a few short spines, 
the mouthparts lighter in color. The body is brownish yellow, with 
rows of slightly darker tubercles, which are thickly studded with 
short brown spines and with a small number of long hairs. Molting 
takes place in five or six days. 

Second-stage larvae, (fig. 3, ^4). — Larva? of the. second stage are 
4 mm. in length. The body is brownish black, except the dorsum 
of the second and third thoracic and the third, fourth, fifth, eighth, 
and ninth abdominal segments, which are yellowish white. The 
dorsum of the sixth and seventh abdominal segments are each pro- 
vided with a retractile tubercle. All tubercles except the latter are 
thickly studded with short brown spines and a few long hairs. After 
feeding five or six days the larva molts again. 

Thira\stage larvae and their hibernating pockets. — In the third 
stage the larvse are G mm. in length. The body is slightly darker 
in color than in the preceding stage and the tubercles are similarly 
arranged. This makes the light colors on the dorsum more con- 
spicuous. The larva? immediately seek shelter and begin the con- 
struction of a small Aveb, in which they hibernate. As a rule these 
are made individually, but occasionally two or sometimes three 
larva? will be found in one of these hibernating pockets. Crevices 
or uneven places in the bark are sought by the larva?, and if the 
cavity is not sufficiently deep a small amount of bark is excavated. 
The cavity is then lined with silk and the larva spins a cover over 
the entrance. After this is accomplished the larva contracts some- 
what in size and is usually about 4 mm. long during the winter and 
somewhat thicker than when it is active. The exposed surfaces of 
these pockets are usually flat and are made so nicely and so closely 
resemble the bark that they are difficult to see. They are circular to 
oval in form, the shape depending on the contour of the crevice in 
which they are made. The length is about 4 mm. 

Many of these pockets have been found on the trunks of large 
Carolina poplar trees (fig. 4), and it was at first thought that the 
bulk of the larva? hibernate near the ground. Examinations have 
shown that the pockets are present high in the trees and on the 



The Satin Moth. 



9 



branches wherever small inequalities in the bark are present. Some 
webs have been taken on branches 60 feet from the "-round and others 




Fig. 4. — Hibernating webs or pockets of small larva? in tree trunk. Several webs are 
encircled with a white line to show their location. There are other webs on this 
bark. 

on small twi<>s and attached to the scars where leaves have fallen. 
While it is probable that most of the larva? remain in the pockets 



10 Department Circular 167, U. S. Dept. of Agriculture. 

during the winter, a few cases have been noted where they have 
deserted them temporarily during the fall. On December 15, 1920. 
Mr. F. H. Mosher reported that a small number of the larva? were 
crawling on the tree trunks at the Medford colony. The weather 
was warm and had been preceded by several warm days. 

The hibernation pockets are very inconspicuous and it is difficult 
to find them even on a tree that is rather heavily infested. This 
being the case the insect could have been brought to this country very 
easily on poplar or willow trees or possibly on other species or on 
bits of infested bark that was mixed in packing material. The color 
of the webs resembles the bark so closely that they can be overlooked 
very easily by the most careful inspector. 

Feeding of the small caterpillars is confined to the epidermis of 
both sides of the leaves, but if the infestation is severe most of the 
foliage on the trees may be denuded so that onty the ribs and frame- 
work remain (fig. 3, A). As most of this feeding occurs during the 
first half of August, many of the leaves that are only partially eaten 
turn brown and drop earlier than normal. While feeding at this 
time of the year does not injure the trees as severely as the defoliation 
by the larger larvae of the previous brood earlier in the season, it 
renders them unsightly and is objectionable, particularly if they are 
planted for ornamental purposes. 

Observations in the field during 1920 indicated that feeding by 
the small larvae was confined principally to different species of pop- 
lar, the Carolina, Lombardy, Balm of Gilead, and silver leaf show- 
ing the most defoliation. In the laboratory experiments willow was 
equally favored, but the small larvae starved rather than feed on' 
alder, apple, gray birch, elm, red oak, and white oak. 

A few small larvae in the laboratory survived on black-oak foliage 
and went into hibernation in a normal manner. This food plant 
may therefore be important, as it is rather common in some sections 
of New England. 

Emergence in the spring has not been observed in this country, 
but the larva? probably come out of the webs and begin feeding as soon 
as sufficient foliage is available. After passing through several 
molts they become full grown about July 1. 

Full-grown larva (fig. 5, C, D). — Larvae when full grown are 45 
to 50 mm. long. The head is bluish black and covered with short 
hairs. The body is black, mottled with white markings of irregular 
shape. These form distinct subdorsal lines on each segment of the 
body and are enlarged into an additional row of somewhat square 
white blotches on the dorsum of each segment. The latter are char- 
acteristic of this species, make it conspicuous, and enable it to be 
readily recognizable. The rows of tubercles are brown, and pro- 
vided with many spines and hairs that are slightly lighter in color. 

After feeding is completed the larvae seek shelter and spin loose 
cocoons. Manv of these are made bv drawing together the edges of 



The Satin Moth. 11 

leaves on the trees. Cocoons are sometimes made in crevices in the 
bark, in rubbish near the base of trees, or on the sides of buildings. 

THE PUP^E. 

The pupa (fig. 5, A, B y E). — The pupa is 20 to 25 mm. in length 
and 5 to 7 mm. wide at the base of the abdomen. It is shiny black, 
densely covered with long white silky hairs, except on the venter, 
where the hairs are scarce. The pupa can easily be seen within the 
cocoon. Pupation was at the maximum by July 6. The time spent 
in the pupal stage averages nine days. 

The first moth was found July 2 and for the next two weeks the 
number increased. 

EUROPEAN RECORDS. 

This species is common in Europe and is mentioned briefly in the 
writings of many European entomologists. It is reported as com- 
mon in England and Ireland and throughout Europe, its range ex- 
tending from Spain to Eastern Asia and as far north as Norway, and 
it is said to be present in the Orient. One authority states that it 
occurs from Lapland to middle Italy and from England to Peking. 
A number of different races have been noted by collectors and are 
distinguished by variations in the coloration of the wings. 

When abundant the moths are said to fty in swarms and to migrate 
long distances. The larvae feed chiefly on poplar and willow, but 
one report states that oak foliage is eaten in England. 

Usually the insect is controlled by parasites and natural enemies 
and is not considered as a pest of first rank. 

Some authorities state that the eggs ordinarily do not hatch until 
spring, but occasionally hatching takes place in July or August if the 
weather is hot, and the small larvae hibernate during the winter. 

DISTRIBUTION IN THE UNITED STATES. 1 

Scouting work to determine the extent to which the insect has 
spread in the United States was begun about the middle of July. 
As it was supposed that the infestation was local and extended for 
only a short distance around the area that was known to be badly 
infested, directions were given the men to examine poplars and 
willows and cover the immediate surroundings of the colony and 
gradually increase the circle if more infested trees were found. Spe- 
cial instructions were given to make careful search for the large 
larvae or pupae, as the finding of these would indicate that the insect 
had been present in this locality during the summer of 1919. So 
many infested trees were found that a new plan of scouting was 
adopted after the first week, the men being instructed to work in the 
cities or towns nearest the infested center and to move out to the 

1 In July, 1920, a small colony of this insect was discovered by Mr. J. D. Tothill, of 
the Entomological Branch. Dominion of Canada, at New Westminster, a suburb of Van- 
couver, British Columbia. 



12 Department Circular 167, U. S. Dept. of Agriculture. 




Fig. 5.— The satin moth: a, pupa, ventral view; b, pupa, lateral view; c, two full- 
grown larva? on bark (a gipsy moth larva is shown on each side of photograph in 
crevices of bark); rf, full-grown larva; e, leaf cut away to show hairy pupa n 
cocoon (larval molt skin shown above pupa), a. b. d, e, enlarged!! rSuced 



The Satin Moth. 



13 



next town as soon as infestation was found. This plan was followed 
until the scouting was completed. Most of the work north and west 
of Boston, with the exception of some towns in the Cape Ann region, 
was done by scouts from the Bureau of Entomology, while the region 
above mentioned and a number of towns south of Boston were scouted 
by regular assistants of the Massachusetts State forester's office. A 
number of town and city gipsy-moth superintendents found and re- 




Fig. 6. — Map of territory infested with the satin moth. 

ported the moth from their municipalities, and this helped materially 
in securing prompt information on the spread of the insect. 

As a result of this work the insect is now known to occur over an 
area of C42 square miles, including 60 towns in Massachusetts and 
4 in New Hampshire. Large larvae or pupa? were found in many 
of the towns indicated on the map (fig. 6) as discovered infested in 
1920, which shows that they were infested the previous year; in fact, 
51 of the G4 towns were so infested. 



14 Department Circular 167, V. S. Dept. of Agriculture. 

Although no infestation was found in a few towns that are sur- 
rounded by infested towns, this does not indicate that the insect is 
not present there. Scouting was done in them rather late to secure 
the best results, and in some of them poplar and willow trees are 
very scarce. 

As is the case with most introduced insects in this region, the 
trend of spread has been toward the north and northeast. This is 
undoubtedly due to the direction of the prevailing wind at the time 
the moths are flying. 

AGE OF THE COLONY. 

The remains of the egg clusters of the satin moth do not adhere to 
the trees very long after hatching takes place. For this reason it is 
very difficult to secure information of value relative to the age of the 
infestations found. 

The worst infestation was in the area along the parkway at the 
Medford-Malden line, and there is no doubt that the insect has been in 
this locality, probably in small numbers, for several yea re. Informa- 
tion from persons residing in this section does not throw much light 
on the subject, as few have been able to distinguish this insect from 
the gipsy moth. The latter has occurred in sufficient numbers in this 
locality during some years to cause much annoyance and complaint. 
Several men who work on the parkway state that the insect was 
present in considerable numbers in this particular locality in 1919, 
and that it was probably there before that time. Mr. C. V. Black- 
burn took a few moths at electric lights in Stoneham in the summer 
of 1919, and he states that they were rather common at the same 
lights in 1920. In Watertown a single Lombardy poplar was found 
badly infested by the local moth superintendent, Mr. Van D. Horton, 
in July, 1920, and Mr. Horton states that the same species was present 
the previous year, but in less numbers. 

v A rather bad infestation was found in Cambridge, and Mr. Dug- 
gan, who had pruned and cared for the trees in 1918, states that the 
satin moth was present at that time, although he supposed that it 
was a native insect. It is probable that the insect has been in this 
country for several years and has only become abundant enough to 
spread very rapidly during the past two or three years. 

While it is impossible to determine the original source of the in- 
festation, it is quite probable that the insect was brought into this 
country during the dormant season in crevices of bark on live or dead 
trees, and bits of infested bark that might be mixed in packing ma- 
terial could easily furnish a means of transporting the insect. As 
previously stated, the hibernating webs of the small larvae concealed 
in the crevices of bark are so inconspicuous as to pass by the most 
careful inspection unnoticed. 



The Satin Moth. 15 

NATURAL ENEMIES. 

The European publications indicate that the satin moth is ex- 
tensively attacked by parasites. It is fortunate, therefore, that some 
of the foreign parasites of the gipsy moth are already well established 
in the infested area in New England. 

Some of the first field observations showed that the imported 
Calosoma beetle {Calosoma sycopha/nta L.), which is one of the 
valuable insect enemies of the gipsy moth, was attacking the larvae 
and pupae of the satin moth. Both the adults and larvae of the beetles 
were doing valiant service- 

A few days later heavy parasitism of the large larvae of the satin 
moth by Compsilura concinnata Meigen, another imported gipsy moth 
enemy, was noted. A number of the records secured indicated that 
it was causing higher mortality than Calosoma. Both of these bene- 
ficial insects are abundant in the central area infested by the gipsy 
moth, and Calosoma was found actively at work in some of the small 
infestations in other towns. 

A small hymenopterous parasite has been reared from the eggs of 
Stilpnotia salkis. It has been determined as Telenomus calif ornicus 
Ashm. by Mr. E. M. Fouts, of the Bureau of Entomology. 

The prospect of control of this insect by parasites is encouraging, 
but it may be necessary to bring in from Europe some of its enemies 
that do not exist here at present. 

REMEDIES. 

Time has not permitted experiments to be conducted to determine 
the most effective remedies. The larvae were so nearly full-grown 
at the time they were first found that spraying did not prove effectual. 
Later, after tlie eggs began to hatch, the trees on the Parkway were 
sprayed with arsenate of lead and a number of trees on private 
grounds were similarly treated. This spraying was partially effec- 
tive, and on some of the trees feeding by the small larvae was greatly 
reduced. The leaves of poplar trees do not hold spray material as 
well as most foliage, hence special care in treatment and the addi- 
tion of a " sticker," such as soft soap, is advisable. 

Application of creosote to the egg clusters will destroy the eggs, 
but it is an expensive method on account of the labor involved and 
is only partially effective, because many egg clusters are deposited on 
small branches and on the leaves and these can not be satisfactorily 
treated. 

It is impractical to require workmen to do extensive climbing in 
poplar trees, as the brittle nature of the wood, and in this region the 
results of the work of borers, particularly the leopard moth (Zeuzera 
pyrma Fab.), render such work hazardous. 

It was hoped that spraying with a contact insecticide while the 
trees were dormant would destroy the hibernating larvae, but as they 



16 Department Circular 167, U. S. Dept. of Agriculture. 

make many of their webs on the twigs and in the tops of the trees, 
this method appears to be impracticable. 

In all probability spring spraying will prove satisfactory, al- 
though no opportunity has been presented to test this method. 

With our present knowledge of the insect, the following treatment 
seems advisable for the season of 1921 : 

Spray poplars and willows as soon as they are in full leaf with 
arsenate of lead at the rate of 10 pounds to 100 gallons of water, 
using soft soap for a " sticker." 

If the infestation is severe, it may be necessary to repeat the opera- 
tion by the middle of June. 

If egg clusters are abundant on the trees in July, spray again with 
the same material about August 1. 

If these suggestions are followed and the amount of spraying is 
governed b}^ the density of infestation, satisfactory results should be 
secured. 

The experience of a full year's work on this insect will probably 
result in a modification of these recommendations. 

PROBABLE IMPORTANCE OF THE SPECIES. 

To predict the ultimate importance of a newly introduced insect 
is rather hazardous. If the feeding is confined to the poplars and 
willows the damage will be greatly restricted. While these trees 
are not considered of prime economic importance, there are certain 
areas where they grow satisfactorily that will not support more use- 
fid species. They arc also effective for ornamental planting in cer- 
tain situations and are valuable for this purpose. Should the species 
change its menu to oak or other trees that are useful and common 
in this country, the status of the problem will be greatly changed. 
Another season will doubtless give the necessaiy opportunity to de- 
termine the destructive possibilities of this insect. 

In the meantime, local moth superintendents, tree wardens, and 
others who are engaged in the protection of trees from insect pests 
should make sure whether or not the insect is present in the terri- 
tory that comes under their supervision and, if so, apply the spray- 
ing program suggested. 

The danger of the shipment of this insect to uninfested territory 
is considerable. As it is practically impossible to guard against this 
possibility by inspecting trees during the dormant season, special 
efforts should be made to examine the favored food plants of the 
insect during July and August to determine whether the insect is 
present. While this applies to the nurseries within the infested area. 
it is necessary that the work be done in the areas surrounding them 
if the danger of spread on shipments is to be eliminated. In cases 
where infestation is found the treatment recommended should be 
applied immediately. 

o 



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